The present invention relates to a method for the production of a dye preparation from madder root.
The root of the madder plant (Rubia tinctorum) contains various dyes among which is alizarin or 1,2-dihydroxyanthraquinone. Alizarin occurs in the fleshy roots as its glycoside, ruberythric acid. The roots have long been a major source of the alizarin dye until it was discovered in 1868 that it can also be chemically synthesized. The chemical synthesis is however becoming more and more expensive and is less desirable from an environmental point of view. Therefore, the madder root was again considered as a useful alternative for the preparation of the dye.
The roots in themselves contain only minor amounts of the dye alizarin. For the formation of alizarin, it is necessary to hydrolyse ruberythric acid, i.e. cleave off the disaccharide group. The roots in addition comprise another glycoside, namely lucidin primeveroside, which contains the same disaccharide as ruberythric acid. During hydrolysis of ruberythric acid lucidin can be formed out of lucidin primeveroside. Lucidin is, however, a known mutagenic agent. Its formation is thus to be avoided.
The hydrolysis of ruberythric acid to alizarin can be done via various routes, namely by means of a strong acid (such as H2SO4), a strong base (such as KOH) or hydrolysis enzymes (so-called hydrolases).
The present inventors have first tested all these methods and came to the following conclusions.
When a suspension of dried roots of madder is refluxed in aqueous 2% H2SO4 solution, this leads to the formation of alizarin, purpurin and xanthopurpurin as the main anthraquinone components and in addition some minor unidentified compounds. Lucidin primeveroside disappears, but it is not known what happens with it.
When a suspension of dried roots of madder in water is refluxed in 2% KOH, alizarin and a broad range of minor components, predominantly anthraquinones, are formed. Because these components are not all identified it is unknown whether mutagenic compounds are present.
According to the literature (T., Masawaki, M. Taya, S. Tone (1996) Selective solvent extraction of ruerythric acid from madder roots and subsequent hydrolysis with xcex2-glucosidase, Journal of Fermentation and Bioengineering 81 (6): 567-569), hydrolysis of the glycosides of madder with xcex2-glucosidases should lead to the formation of alizarin and lucidin. The present inventors have repeated the experiment but did not succeed in obtaining alizarin and lucidin. Using commercially available enzyme preparations containing xylenase and glycosidase activity on a madder suspension (in which endogenous madder enzymes were inactivated) did lead to alizarin and lucidin. But this method is not suitable either because it would need an additional step to get rid of the mutagenic lucidin.
It is therefore the object of the present invention to provide a new method for producing a dye preparation from madder root, in which preparation the amount of the dye alizarin is increased but which method does not lead to the formation of the mutagenic by-product lucidin in the preparation.
This is achieved by the invention by a method, comprising extraction of madder root in water at a temperature below 70-75xc2x0 C., preferably below 65xc2x0 C., while providing oxygen to the reaction mixture of madder root in water. In the research that led to the present invention it was established that the hydrolysis of ruberythric acid can be performed by endogenous madder root enzymes. In this enzymatic process also nordamnacanthal is formed out of lucidin primeveroside. It was found that when insufficient or no oxygen is present lucidin is formed instead of the harmless nordamnacanthal. Thus, for avoiding the formation of lucidin, the presence of oxygen is mandatory.
The present invention is a method, comprising extraction of madder root in water at a temperature below 70-75xc2x0 C., preferably below 65xc2x0 C., while providing oxygen to the reaction mixture of madder root in water. In the research that led to the present invention it was established that the hydrolysis of ruberythric acid can be performed by endogenous madder root enzymes. In this enzymatic process also nordamnacanthal is formed out of lucidin primeveroside. It was found that when insufficient or no oxygen is present lucidin is formed instead of the harmless nordamnacanthal. Thus, for avoiding the formation of lucidin, the presence of oxygen is mandatory.
It was found that the conversion of the dye components in madder roots by endogenous enzymes is optimal when the temperature lies between 35 and 65xc2x0 C. at a reaction time of about 30 minutes. At higher reaction times of for example 4 hours, the temperature may be even lower, to as low as 0xc2x0 C.
Because the conversion of lucidin primeveroside is enzymatic it is important to keep the reaction temperature below a value that would lead to denaturation of the enzymes. Preferably the temperature is kept below 65xc2x0 C. At a higher temperature, the enzymes start to be denatured. Although the reaction will still go on for some time, the conversion is not complete and therefore not useful for the preparation of the dye.
It was found that the transformation of ruberythric acid and lucidin primeveroside into alizarin and lucidin, respectively, can be performed in demiwater, ultra-pure water and even tap water. The optimum pH range varies with the extraction time. The optimum pH for 30 minutes reaction time is about 6. At higher and lower pH""s the percentage conversion lowers rapidly. At 4 hours the pH optimum lies between 6 and 8.
The provision of oxygen can be achieved by simple stirring of the reaction mixture or the more direct introduction of air or oxygen into the reaction mixture. It is preferred to enrich water to be used in the reaction with oxygen prior to the addition of the madder. It is furthermore preferred to also add oxygen during the addition of the madder and during the reaction. The availability of oxygen can be improved by increasing the contact area of the reaction water with the surrounding air or added oxygen.
The absence of lucidin in the final product can be confirmed by means of HPLC.
The term xe2x80x9cmadder rootxe2x80x9d as used in this application refers to any from of madder root, such as fresh roots, dried roots, root powder etc.
The present invention is further illustrated in the following examples.